Computer Lockups

For some time now i've been having random system lockups since having my computer repaired around the first of the year. It'll run okay for a few min or so then it'll just freeze up. I've tried reinstalling drivers and even the OS, even tried using a different OS. A friend took it home and said he tried it out and everything ran fine with no problems at all but as soon as i got it back the same thing keeps happening. I've also noticed that the computer does not keep time very well (the friend put a new lithium battery in thinking that was the only problem) He's also claimed that its probably just a USB issue. So i tried an older Mouse/keybard (non USB). But despite all the Driver Updates and OS installs still the problem has yet to be resolved. Any help would be appreciated.

AMD Athlon(tm) 64 X2 Dual Core Processor 3800+
Winfast NF4UK8AA
1Gig Ram
PHILIPS DVD+RW-D28
Lite-On LTN486S 48x Max
Maxtor 300gb HD
Targus Lazer Mouse
Saitek Keyboard
OS - Originally WinXP Pro, Now Vista Home Premium